Network Working Group S. Kitterman
Internet-Draft fTLD Registry Services
Updates: 7489 (if approved) November 21, 2018
Intended status: Informational
Expires: May 25, 2019

DMARC (Domain-based Message Authentication, Reporting, and Conformance) Extension For PSDs (Public Suffix Domains)


DMARC (Domain-based Message Authentication, Reporting, and Conformance) is a scalable mechanism by which a mail-originating organization can express domain-level policies and preferences for message validation, disposition, and reporting, that a mail-receiving organization can use to improve mail handling. DMARC policies can be applied at the individual domain level or for a set of domains at the organizational level. The design of DMARC precludes grouping policies for a set of domains above the organizational level, such as TLDs (Top Level Domains). These types of domains (which are not all at the top level of the DNS tree) can be collectively referred to as Public Suffix Domains (PSDs). For the subset of PSDs that require DMARC usage, this memo describes an extension to DMARC to enable DMARC functionality for such domains.

Status of This Memo

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This Internet-Draft will expire on May 25, 2019.

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Table of Contents

1. Introduction

DMARC provides a mechanism for publishing organizational policy information to email receivers. DMARC allows policy to be specified for both individual domains and sets of domains within a single organization. For domains above the organizational level in the DNS tree, policy can only be published for the exact domain. There is no method available to such domains to express lower level policy or receive feedback reporting for sets of domains. This prevents policy application to non-existent domains and identification of domain abuse in email, which can be important for brand and consumer protection.


As an example, imagine a country code TLD (ccTLD) which has public subdomains for government and commercial use (.gov.example and .com.example). Within the .gov.example public suffix, use of DMARC has been mandated and .gov.example has published its own DMARC record:

This memo provides a simple extension to DMARC to allow operators of Public Suffix Domains (PSDs) to express policy for groups of subdomains, extends the DMARC policy query functionality to detect and process such a policy, describes receiver feedback for such policies, and provides controls to mitigate potential privacy considerations associated with this extension.

There are two types of Public Suffix Operators (PSOs) for which this extension would be useful and appropriate:

Due to the design of DMARC and the nature of the Internet email architecture, there are interoperability issues associated with DMARC deployment. These are discussed in Interoperability Issues between DMARC and Indirect Email Flows. These issues are not applicable to PSDs, since they (e.g., the ".gov.example" used above) do not send mail.

DMARC, by design, does not support usage by PSD operators. For PSDs that require use of DMARC, an extension of DMARC reporting and enforcement capability is needed for PSD operators to effectively manage and monitor implementation of PSD requirements.

2. Terminology and Definitions

This section defines terms used in the rest of the document.

2.1. Conventions Used in This Document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

2.2. Public Suffix Domain (PSD)

The global Internet Domain Name System (DNS) is documented in numerous Requests for Comment (RFC). It defines a tree of names starting with root, ".", immediately below which are Top Level Domain names such as ".com" and ".us". They are not available for private registration. In many cases the public portion of the DNS tree is more than one level deep. PSD DMARC includes all public domains above the organizational level in the tree, e.g., "".

2.3. Longest PSD

Organizational Domain (DMARC Section 3.2) with one label removed.

2.4. Public Suffix Operator (PSO)

A Public Suffix Operator manages operations within their PSD.

2.5. PSO Controlled Domain Names

PSO Controlled Domain Names are names in the DNS that are managed by a PSO and are not available for use as Organizational Domains (the term Organizational Domains is defined in DMARC Section 3.2). Depending on PSD policy, these will have one (e.g., ".com") or more (e.g., "") name components.

2.6. Non-existent Domains

For DMARC purposes, a non-existent domain is a domain name that publishes none of A, AAAA, or MX records that the receiver is willing to accept. This is a broader definition than that in NXDOMAIN.

3. PSD DMARC Updates to DMARC Requirements

This document updates DMARC as follows:

3.1. General Updates

References to "Domain Owners" also apply to PSOs.

3.2. Section 6.1 DMARC Policy Record

PSD DMARC records are published as a subdomain of the PSD. For the PSD ".example", the PSO would post DMARC policy in a TXT record at "_dmarc.example".

3.3. Section 6.5. Domain Owner Actions

In addition to the DMARC domain owner actions, PSOs will need to update the "DMARC Public Suffix Domain (PSD) Registry". This registry is defined in Section 6.1.

3.4. Section 6.6.3. Policy Discovery

A new step between step 3 and 4 is added:

If the set is now empty and the longest PSD of the Organizational Domain is listed in the DMARC PSD Registry (defined in Section 6.1), the Mail Receiver MUST query the DNS for a DMARC TXT record at the DNS domain matching the longest PSD in place of the RFC5322.From domain in the message (if different). A possibly empty set of records is returned.

As an example, for a message with the Organizational Domain of "", the query for PSD DMARC would use "" as the longest PSD. The receiver would check to see if that PSD is listed in the DMARC PSD Registry, and if so, perform the policy lookup at "".

Note: Because the PSD policy query comes after the Organizational Domain policy query, PSD policy is not used for Organizational domains that have published a DMARC policy. Specifically, this is not a mechanism to provide feedback addresses (RUA/RUF) when an Organizational Domain has declined to do so.

3.5. Section 7. DMARC Feedback

Operational note for PSD DMARC: For PSOs, feedback for non-existent domains is desired and useful. Because of the constraints on PSD DMARC scope, there are no significant privacy considerations associated with this reporting (See Section 4).

4. Privacy Considerations

This document does not significantly change the Privacy Considerations of [RFC7489].

4.1. Feedback leakage

Providing feedback reporting to PSOs can, in some cases, create leakage of information outside of an organization to the PSO. There are roughly three cases to consider:

PSOs will receive feedback on non-existent domains, which may be similar to existing Organizational Domains. Feedback related to such cousin domains have a small risk of carrying information related to an actual Organizational Domain. To minimize this potential concern, PSD DMARC feedback is best limited to Aggregate Reports. Feedback Reports carry more detailed information and present a greater risk.

5. Security Considerations

This document does not change the Security Considerations of [RFC7489].

6. IANA Considerations

This section describes actions requested to be completed by IANA.

6.1. DMARC Public Suffix Domain (PSD) Registry

IANA is requested to create a new DMARC Public Suffix Domain (PSD) Registry within the Domain-based Message Authentication, Reporting, and Conformance (DMARC) Parameters Registry.

Names of PSDs participating in PSD DMARC must be registered with IANA in this new sub-registry. New entries are assigned only for PSDs that require use of DMARC. The requirement has to be documented in a manner that satisfies the terms of Expert Review, per [RFC5226]. The Designated Expert needs to confirm that provided documentation adequately describes PSD policy to require domain owners to use DMARC or that all domain owners are part of a single organization with the PSO.

|    PSD      | Reference      | Status        |
| .bank       | this document  | current       |
| .insurance  | this document  | current       |
|     | this document  | current       |

The initial set of entries in this registry is as follows:

7. References

7.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC7489] Kucherawy, M. and E. Zwicky, "Domain-based Message Authentication, Reporting, and Conformance (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.

7.2. Informative References

[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 5226, DOI 10.17487/RFC5226, May 2008.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, DOI 10.17487/RFC5598, July 2009.
[RFC7960] Martin, F., Lear, E., Draegen. Ed., T., Zwicky, E. and K. Andersen, "Interoperability Issues between Domain-based Message Authentication, Reporting, and Conformance (DMARC) and Indirect Email Flows", RFC 7960, DOI 10.17487/RFC7960, September 2016.
[RFC8020] Bortzmeyer, S. and S. Huque, "NXDOMAIN: There Really Is Nothing Underneath", RFC 8020, DOI 10.17487/RFC8020, November 2016.



Author's Address

Scott Kitterman fTLD Registry Services 600 13th Street, NW, Suite 400 Washington, DC 20005 United States of America Phone: +1 301 325-5475 EMail: